1. Field of the Invention
The present invention relates to a power control unit protection apparatus for preventing thermal breakdown due to overheating of the power supply control unit, including the power supply transformer, by evaluating the load supply power loss (current) and amplifier power loss (temperature), and limiting the power supply after waiting a specified delay time.
2. Description of the Prior Art
Power control unit protection apparatuses provide an important protection function for power supply semiconductors and power supply transformers used in the power supply control field, and are available in many different forms.
A conventional power control unit protection apparatus of this type is shown in FIG. 5 and described below with reference to the figures.
As shown in FIG. 5, this power control unit protection apparatus comprises a power supply transformer 1; a first input terminal 2 on the primary side of the power supply transformer 1; a second input terminal 3 on the primary side of the power supply transformer 1; a temperature fuse 4 built in to the power supply transformer 1; a high voltage rectifier 5; a first output terminal 6 at the high voltage tap of the power supply transformer 1; a second output terminal 7 at the high voltage tap of the power supply transformer 1; a positive output terminal 8 to the high voltage rectifier 5; a negative output terminal 9 to the high voltage rectifier 5; a positive high voltage rectification capacitor 10; a negative high voltage rectification capacitor 11; a low voltage rectifier 12; a first output terminal 13 at the low voltage tap of the power supply transformer 1; a second output terminal 14 at the low voltage tap of the power supply transformer 1; a positive output terminal 15 to the low voltage rectifier 12; a negative output terminal 16 to the low voltage rectifier 12; a positive low voltage rectification capacitor 17; a negative low voltage rectification capacitor 18; a dual power supply switching controller 19; a dual power supply switching controller signal input terminal 20; complementary output transistors 21 and 22; a positive high voltage power supply control output transistor 23; a negative high voltage power supply control output transistor 24; a resistor 25 and Zener diode 26 for generating the bias current of the positive high voltage power supply control output transistor 23; a positive switching diode 27; a resistor 28 and Zener diode 29 for generating the bias current of the negative high voltage power supply control output transistor 24; a negative switching diode 30; an output terminal 31; a load resistor 32 such as a speaker; a ground terminal 33; a dual power supply switching controller temperature detector 34; a temperature detector output terminal 35; and a high voltage power supply interrupt controller 36.
The power control unit protection apparatus thus comprised operates as follows.
When an input signal is applied to the dual power supply switching controller signal input terminal 20, the complementary output transistors 21 and 22 operate as emitter followers, and the voltage at the output terminal 31 is approximately equal to the input. When the absolute value of the output voltage is sufficiently lower than the positive output terminal 15 voltage, the current flowing to the speaker 32 passes in the positive half cycle through the positive output terminal 15 of the low voltage rectifier, the positive switching diode 27, output transistor 21, and to the speaker 32. When the absolute value of the output voltage is greater than the positive output terminal 15 voltage, the current flowing to the speaker 32 passes in the positive half cycle through-the positive output terminal 8 of the high voltage rectifier, the positive high voltage power supply control output transistor 23, output transistor 21, and to the speaker 32. The wave form at this time is shown in FIG. 6.
The difference between the output voltage e.sub.0 at the output terminal and the collector voltages Vc and Vc' of the transistors 21 and 22 is the voltage drop at the biasing Zener diodes 26 and 29 (FIG. 5) if the base emitter voltage V.sub.BE of transistors 23 and 24 is ignored, and a bias voltage is constantly applied so that the non-linear (saturation) range of the output transistor 21 is not used.
When the absolute value of the output voltage at the output terminal 31 is sufficiently lower than the low voltage rectifier output terminal 15, the power supply from the power supply transformer 1 to the dual power supply switching controller operating as described above is supplied through the first and second output terminals 13 and 14 on the low voltage tap of the power supply transformer 1. When the absolute value of the output voltage is greater than the positive output terminal 15 voltage, power is supplied through the first and second output terminals 6 and 7 on the high voltage tap of the power supply transformer 1.
The power supply loss characteristic of a typical dual power supply switching controller is shown in FIG. 7, wherein the output voltage amplitude appearing at the output terminal 31 is plotted on the X axis, and the combined power loss values of the dual power supply switching controller transistors 21 and 23 (22, 24) are plotted on the Y axis. VTH in FIG. 7 indicates the voltage point at which the absolute value of the output voltage exceeds the voltage of the low voltage rectifier output terminal 15.
The dotted line curve in FIG. 7 indicates the power loss when using a single power supply controller, i.e., there is no power supply from terminals 15 and 16 in FIG. 5 and power is supplied only from terminals 8 and 9.
As will be known from FIG. 7, power loss with a dual power supply switching controller is less at all amplitude levels than the power loss with a single power supply controller (known as a Class B controller). In other words, more efficient power control is possible using a dual power supply switching controller. Efficiency is particularly high when the power supply is input from a low voltage rectifier output terminal, making it possible to supply power using a power supply transformer with a smaller core than a power supply transformer feeding a single power supply controller.
When the dual power supply switching controller temperature detector 34, which detects the average power loss (temperature) of the dual power supply switching controller 19, detects the specified temperature, the high voltage power supply interrupt controller 36 operates to shut off the high voltage power supply control output transistors 23 and 24, thereby protecting against excessive power loss in the power supply controller.
However, while the conventional power control unit protection apparatus described above enables temperature protection of the dual power supply switching controller, there is no effective means of providing similar temperature protection to the power supply transformer. The problem here is that the power loss of the dual power supply switching controller is not proportional to the load power loss. As a result, reducing the size of the power supply transformer designed for maximum efficiency in the dual power supply switching controller is made difficult due to internal damage to the power supply transformer, typically blowing the temperature fuse 4, if the power supply is maintained for a certain period during an excessive load power loss state.